Semiconductor substrate for solar cell and solar cell   

Abstract: A solar cell include a polycrystalline semiconductor substrate of a p-type, an emitter region of an n-type and forming a p-n junction with the polycrystalline semiconductor substrate, a first electrode connected to the emitter region, and a second electrode connected to the polycrystalline semiconductor substrate, wherein the polycrystalline semiconductor substrate has a pure p-type impurity concentration of substantially 7.2×1015/cm3 to 3.5×1016/cm3.

This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0101512, filed in the Korean Intellectual Property Office on Oct. 18, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

Embodiments of the invention relate to a semiconductor substrate for a solar cell and the solar cell.

(b) Description of the Related Art

Recently, as existing energy sources such as petroleum and coal are expected to be depleted, interests in alternative energy sources for replacing the existing energy sources are increasing. Among the alternative energy sources, solar cells for generating electric energy from solar energy have been particularly spotlighted.

A solar cell generally includes semiconductor parts that have different conductive types, such as a p-type and an n-type, and form a p-n junction, and electrodes respectively connected to the semiconductor parts of the different conductive types.

When light is incident on the solar cell, a plurality of electron-holes pairs are generated in the semiconductor parts. By a p-n junction, the electrons move to the n-type semiconductor part and the holes move to the p-type semiconductor part, and then the electrons and holes are collected by the electrodes electrically connected to the n-type semiconductor part and the p-type semiconductor part, respectively. The electrodes are connected to each other using electric wires to thereby obtain electric power.

SUMMARY

According to an aspect of the invention, a semiconductor substrate for a solar cell made of a semiconductor of a p-type includes a first impurity of the p-type, a second impurity of the p-type, and a third impurity of an n-type, wherein a pure p-type impurity concentration of the semiconductor substrate is substantially 7.2×1015/cm3 to 3.5×1016/cm3 and wherein the pure impurity concentration is obtained by subtracting a concentration of the third impurity from a sum of a concentration of the first impurity and a concentration of the second impurity.

The concentration of the second impurity may be substantially 1×1013/cm3 to 5.5×1015/cm3.

The sum of the concentration of the first impurity and the concentration of the second impurity may be substantially 3.5×1016/cm3 or less.

The concentration of the third impurity may be substantially 2.8×1016/cm3 or less.

The first impurity may be boron (B), and the second impurity may be at least one of the aluminum (Al) and gallium (Ga), and the third impurity may be phosphorus (P).

The semiconductor substrate may be a polycrystalline silicon substrate.

The semiconductor substrate may further include iron (Fe).

The concentration of the iron (Fe) may be substantially 6×1015/cm3 or less.

According to another aspect of the invention, a solar cell includes a polycrystalline semiconductor substrate of a p-type, an emitter region of an n-type and forming a p-n junction with the polycrystalline semiconductor substrate, a first electrode connected to the emitter region, and a second electrode connected to the polycrystalline semiconductor substrate, wherein the polycrystalline semiconductor substrate has a pure p-type impurity concentration of substantially 7.2×1015/cm3 to 3.5×1016/cm3.

The solar cell may have a breakdown voltage of substantially 12V to 41V.

The polycrystalline semiconductor substrate may contain boron (B), at least one of aluminum (Al) and gallium (Ga), and phosphorous (P).

The concentration of the aluminum (Al) may be substantially 1×1013/cm3 to 5.5×1015/cm3.

A sum of a concentration of boron (B) and a concentration of at least one of aluminum (Al) and gallium (Ga) may be substantially 3.5×1016/cm3 or less.

The concentration of the phosphorus (P) may be substantially 2.8×1016/cm3 or less.

The polycrystalline semiconductor substrate may further include iron (Fe).

The concentration of the iron (Fe) may have a concentration of substantially 6×1015/cm3 or less.

The polycrystalline semiconductor substrate may include a first impurity of the p-type, a second impurity of the p-type, and a third impurity of the n-type, wherein the pure impurity concentration may be obtained by subtracting a concentration of the third impurity from a sum of a concentration of the first impurity and a concentration of the second impurity.

The concentration of the second impurity may be substantially 1×1013/cm3 to 5.5×1015/cm3.

The sum of the concentration of the first impurity and the concentration of the second impurity may be substantially 3.5×1016/cm3 or less, and the concentration of the third impurity may be substantially 2.8×1016/cm3 or less.

The first impurity may be boron (B), the second impurity may be at least one of aluminum (Al) and gallium (Ga), and the third impurity may be phosphorus (P).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:

FIG. 1 is a graph showing a reverse-bias voltage according to a variation of a pure impurity concentration of impurities contained in a substrate;

FIG. 2 is a partial perspective view of a solar cell according to an example embodiment of the invention; and

FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2

DETAILED DESCRIPTION

The invention will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the inventions are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Further, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being “entirely” on another element, it may be on the entire surface of the other element and may not be on a portion of an edge of the other element.

Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings.

First, a substrate, that is, a semiconductor substrate, for a solar cell according to the embodiment is described.

A semiconductor substrate of a general solar cell includes a p-type semiconductor portion and an n-type semiconductor portion to form a p-n junction, and electrodes are connected to the p-type and n-type semiconductor portions of the semiconductor structure, respectively. Then, when light is incident on the semiconductor substrate, a plurality of electron-holes pairs are generated and the electrons and holes move to electrodes due to the p-n junction Thereby, the electrons and holes are outputs to the electrodes to obtain electric power. In this instance, a voltage obtained by one solar cell is generally about 0.6V. The semiconductor substrate for the solar cell is a single crystal silicon substrate or a polycrystalline silicon substrate.

In general, the solar cells generating electricity using irradiated light are arranged in a matrix structure, for example, a 10×6 matrix structure, and the solar cells of the predetermined number (for example, 60) are electrically connected in series to each other to form one solar cell module. Thereby, at least one solar cell module is installed in a space to construct a photovoltaic system.

When in some of the installed solar cell modules, light is not normally incident onto at least one solar cell or a shade is generated in at least one solar cell by the clouds, the leaves, or the dusts, etc., each of the voltage and the current generated from the at least one solar cell does not have a normal magnitude and is thereby less than the normal magnitude. Hereinafter, the solar cell not generating the voltage and current of the normal magnitude is referred to as an abnormal solar cell, and a solar cell which normally receives light onto the entire incident surface thereof and generates the voltage and current of normal magnitudes is referred to as a normal solar cell. When at least one abnormal solar cell exists in a solar cell module, a sum of voltages generated from the remaining normal solar cells is applied to the at least one abnormal solar cell as a reverse-bias voltage.

Thus, a reverse current based on the reverse-bias voltage flows through the abnormal solar cell supplied with the reverse-bias voltage, and when the reverse-bias voltage exceeds a threshold voltage, that is, a breakdown voltage, the reverse current flowing through the abnormal current rapidly increases. In this instance, the phenomenon of rapid increment (or increase) of the reverse current is referred to as a breakdown phenomenon.

As described above, when the reverse current flows through a solar cell, heat is generated in the solar cell to deteriorate the solar cell and to thereby reduce the efficiency and the lifetime of the solar cell. Accordingly, power generated from the solar cell module also decreases.

A magnitude of the breakdown voltage causing the rapid increment (or increase) of the reverse current is varied in accordance with concentrations of impurities contained in the substrate, that is, the semiconductor substrate, of the solar cell.

For protecting the at least one abnormal solar cell and the solar cell module that includes the at least one abnormal solar cell from the reverse-bias voltage, the solar cell module includes at least one bypass diode.

However, in a commercial solar cell module, one bypass diode is not installed for every one solar cell, but rather, one bypass diode is installed for every two solar cell columns that are adjacent to each other, and therefore, one bypass diode is installed for a pair of solar cell columns. For example, when the plurality of solar cells are arranged in a 10×6 matrix structure, for example, the total number of the bypass diodes is 3. Thereby, the number of solar cells protected by one bypass diode is, for example, 20, which is the entire number of solar cells existing in two solar cell columns.

Thus, in one solar cell column in which a plurality of solar cells are connected in series, when the number of the abnormal solar cell is one, a voltage of a magnitude corresponding to a sum of voltages generated from the remaining normal solar cells is applied to the abnormal solar cell as a reverse-bias voltage.

For example, when 20 solar cells each of which outputs a voltage of about 0.6V in a normal state are connected in series in two solar cell columns and one of 20 solar cells is a abnormal solar cell, a magnitude of a reverse-bias voltage applied to the one abnormal solar cell is about 11.4V (=0.6V×19 solar cells) because a voltage outputted from each normal solar cell is about 0.6V. Accordingly, in this instance, all of the voltages outputted from 19 solar cells among the total of 20 solar cells protected by one bypass diode are applied to the one abnormal solar cell as the reverse voltage. Thus, in the solar cell module of the 10×6 matrix structure, the maximum reverse bias-voltage to be applied to each solar cell is about 11.4V. Although discussed in terms of a 10×6 matrix structure, embodiments of the invention may include a varying number of matrix structures.

The breakdown phenomenon of the solar cell due to the reverse-bias voltage is varied depending on concentrations of impurities in the substrate and an internal field in a depletion layer generated from the p-n junction. The internal field increases further as application and increment of the reverse bias voltage occurs, and thereby, by generating the tunneling phenomenon and the avalanche breakdown phenomenon of electrons from the p-type substrate, rapid reverse current increment at a specific reverse-bias voltage is caused.

A substrate for manufacturing the solar cell contains impurities in a raw material (for example, SiO2) for manufacturing a semiconductor wafer to be used as the substrate for the solar cell or impurities [for example, phosphorus (P), aluminum (Al), gallium (Ga), iron (Fe), etc.] doped for improving characteristics of the substrate as well as an impurity [for example, boron (B)] artificially doped for a p-type conductivity.

When the substrate of the solar cell is of a p-type, boron (B) and at least one of aluminum (Al) and gallium (Ga) is a p-type impurity that is of the same conductivity type as the substrate, and phosphorus (P) is an n-type impurity that is of an opposite conductivity type as the substrate. Thus, a concentration of the p-type impurity in the p-type substrate is changed or modified by the presence of the n-type impurity. If desired, gallium (Ga) need not be injected (or included) in the substrate. Thus, when gallium (Ga) is injected (or included) in the substrate, the p-type impurities are boron (B), aluminum (Al) and gallium (Ga), but when gallium (Ga) is not injected (or included) in the substrate, the p-type impurities are boron (B) and aluminum (Al).

As described above, the breakdown voltage of the substrate for the solar cell is influenced by the internal field from the p-n junction of the substrate (which is a p-type) and an emitter region (which is an n-type). As the concentration of the p-type impurity increases, an initial internal field (that is, an internal field when the reverse-bias voltage is 0V) increases, and when a reverse-bias voltage is applied to the substrate for the solar cell in a state that the initial internal field is increasing, the depletion layer due to the p-n junction further increases.

That is, when the reverse-bias voltage is applied to the solar cell, a (+) voltage is applied to the n-type emitter region that is positioned towards a front surface of the p-type substrate, on which light is incident, and a (−) voltage is applied to a back surface of the p-type substrate, which is positioned opposite to the front surface of the substrate.

Thus, electrons (e−) move toward the emitter region by the (+) voltage applied to the emitter region, and holes (h+) move toward the back surface of the substrate by the (−) voltage applied to the back surface of the substrate, and thereby, the depletion layer in the p-n junction further increases when the reverse-bias voltage is applied to the solar cell. When the depletion layer increases, the internal field of the substrate for the solar cell also increases, and thereby, when a magnitude of the reverse-bias voltage applied to the solar cell reaches a specific threshold value, the breakdown phenomenon occurs.

Thereby, elements that affect the reverse-bias voltage of the solar cell are impurity concentrations, and the internal field of the substrate from the p-n junction.

In the embodiment, by increasing a magnitude of the breakdown voltage of the substrate for the solar cell, which is a p-type polycrystalline silicon substrate, for example, to about 12V and more, even though the largest reverse-bias voltage (the maximum reverse-bias voltage) (e.g., about 11.4V) is applied to a solar cell of a solar cell module, it prevents or reduces the occurrence of the breakdown phenomenon.

As described, since the magnitude of the breakdown voltage is affected by the impurity concentrations of the substrate, the impurity concentrations of the substrate which has been sliced from the ingot for the solar cell is controlled considering the influence of the internal field due to the variation of the depletion layer in the forming of the p-n junction. In this instance, the magnitude of the breakdown voltage of the substrate is controlled in consideration of not only the concentration of boron, which is a p-type impurity, injected (or included) for obtaining the conductivity type of the substrate, but also the concentrations of the impurities, that is, phosphorous (P) which is an n-type impurity, and at least one of aluminum (Al) and gallium (Ga) which are also p-type impurities. The phosphorous (P), aluminum (Al) and gallium (Ga) are contained (or included) in the raw material of the substrate, as described.

Thus, the impurity concentration Ct of the substrate (sliced from the ingot) that affects the p-type of the substrate is controlled in consideration of all of a concentration C1 of the p-type impurity such as boron (B), a concentration C2 of the n-type impurity such as phosphorus (P), and a concentration C3 of at least one of aluminum (Al) and gallium (Ga) which are p-type impurities.

The impurity concentration Ct of the substrate that affects the p-type of the substrate is obtained by subtracting the concentration C2 of the n-type impurity [phosphorous (P)] from the total concentration (C1+C3) of the p-type impurities, that is, a sum (C+C3) of the concentration C1 of boron (B) and the concentration C3 of at least one of aluminum (Al) and gallium (Ga). Thereby, Ct=(C1+C3)−C2. In this instance, the impurity concentration [Ct=(C1+C3)−C2] of the p-type substrate in consideration of all of the p-type impurities and the n-type impurity is referred to as ‘a pure impurity concentration.’ That is, the pure impurity concentration is a pure p-type impurity concentration. In another word, the pure impurity concentration is an effective purity of the p-type impurities.

As a result, the increment of the pure p-type impurity concentration Ct increase the internal field into the depletion layer of the p-n junction, and when the reverse-bias voltage is applied to the substrate for the solar cell, the internal field largely or greatly increases to cause the breakdown phenomenon.

In the embodiment of the invention, a changed reference voltage (about 12V) of the breakdown voltage is a value for preventing the occurrence of the breakdown phenomenon even though the largest reverse-bias voltage (about 11.4V) capable of being applied to one solar cell is applied to one abnormal solar cell of a solar cell module which includes a plurality of solar cells arranged in the 10×6 matrix structure. Accordingly, the embodiment of the invention is directed to changing the magnitude of the breakdown voltage by controlling the concentrations of impurities contained in the substrate in order not to cause the breakdown phenomenon even though the largest reverse-bias voltage is applied to one abnormal solar cell.

In consideration of the internal field which is changed by the reverse-bias voltage in the formation of the p-n junction, the concentrations of the impurities in the substrate for obtaining the breakdown voltage of about 12V and greater is discussed below. In an embodiment of the invention, the substrate is a substrate that is sliced from an ingot for manufacturing the solar cell.

The pure impurity concentration Ct of the sliced substrate for the solar cell is about 7.2×1015/cm3 to 3.5×1016/cm3. In this instance, the concentration of aluminum (Al) is about 1×1013/cm3 to 5.5×1015/cm3, the concentration (or the sum concentration) (C1+C3) of the concentration C1 of the p-type impurity such as boron (B) and the concentration C3 of at least one of aluminum (Al) and gallium (Ga) is about 3.5×1016/cm3 or less. When the concentration (C1+C3) of the p-type impurities is 3.5×1016/cm3 or less, the concentration C2 of phosphorous (P) which is the n-type impurity is about 2.8×1016/cm3 or less. The concentration of iron (Fe) is about 6×1015/cm3 or less.

When the pure impurity concentration Ct of the substrate is about 3.5×1016/cm3 or less, the minimum value of the breakdown voltage is about 12V, and when the pure impurity concentration Ct of the substrate is about 7.2×1015/cm3 or more, the substrate stably maintains the p-type (or the p-type characteristic).

As described above, when a solar cell is manufactured using the substrate having the concentration of the p-type impurities and the concentration of the n-type impurities, a theoretical breakdown voltage with respect to the solar cell is as described with reference to FIG. 1.

FIG. 1 shows a range of the theoretical breakdown voltages with respect to the pure impurity concentrations, in the p-type polycrystalline silicon substrate having band gap energy of about 0.7 eV to 0.76 eV. The lowest limit of each theoretical breakdown voltage is a theoretical value in an instance that the polycrystalline silicon substrate has the band gap energy of about 0.7 eV, and the uppermost limit of each theoretical breakdown voltage is a theoretical value in an instance that the polycrystalline silicon substrate has the band gap energy of about 0.76 eV.

The theoretical breakdown voltage is calculated by [Equation 1].

E  ( x = 0 ) = - [ 2   q ε   Si  ( NaNd Na + Nd )  ( Vbi - Va ) ] 1 2 [ Equation   1 ]

Here, E(x) is an internal field value, q is quantity of electric charge, that is, 1.6×1019 (coulomb), Na is a pure impurity concentration, Nd is a doped concentration of an emitter region, Vbi is a built-in potential, εSi is permittivity of silicon and Va is a breakdown voltage.

In [Equation 1], the breakdown voltage (Va) is obtained by [Equation 2].

 Va  = E 2 · ε   Si 2   q  ( Na + Nd NaNd ) - Vbi [ Equation 

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